The present invention relates to a spray apparatus for electrostatic powder coating and, more particularly to a spray gun having a main tool which is adapted to be used with several different attachment pieces, each operating on a different spraying principle.
The main tool has a powder delivery channel, a mouth on the downstream end of the delivery channel effective for atomizing the powder, a powder feed line for feeding pneumatically conveyed powder into the initial upstream section of the delivery channel, and a gas feed line through which gas is guided into the delivery channel along a discharge electrode which can be connected to ground potential.
A spray apparatus of the above type is known from Federal Republic of Germany Pat. No. 23 47 491. The wall of the delivery channel is constructed of electrically insulating material against which particles of powder rub to thereby become electrically charged. In the process, however, electric charges collect on the channel wall and these charges impede the process of electrically charging the powder particles. To solve the problem, a discharge electrode extends within the delivery channel into the powder/gas stream and serves to remove the accumulated charge from the channel wall via the discharge electrode to a ground potential. The gas in the gas feed line flows over the discharge electrode to assure that powder particles do not attach themselves to the electrode.
A similar apparatus is also described in Federal Republic of Germany Pat. No. 22 03 351. Moreover, German Democratic Republic Pat. No. 134 841 also describes a spray apparatus of the above type in which a flow guide member of electrically insulating material is arranged within the delivery channel causing the powder to charge electrically by rubbing both on the channel wall and on the surface of the flow guide member.
Federal Republic of Germany OS No. 27 56 009 (corresponding to U.S. Pat. No. 4 090 666) describes a spray apparatus that does not have a discharge electrode. Gas is introduced into the delivery channel, via a gas feed line that extends coaxially to the powder/gas stream of the pneumatically conveyed powder in a manner that is effective for driving the particles of powder together with the gas in the gas feed line radially outward and against the wall of the channel. As a result, more particles of powder rub along the wall of the channel with stronger frictional pressure. The powder as a whole is therefore more strongly charged
There are also other spray devices in which the powder is electrically charged by high voltage electrodes which are located in the flow path of the powder and which are connected to a high-voltage terminal of a high-voltage generator circuit. In this type of spray gun, a distinction is made between so-called "internal charging", in which the electrode is located upstream from the place where the atomization of the powder begins, and so-called "external charging," in which the electrode is located in a region where the powder is being atomized or has already been atomized. "Atomization" refers to the process whereby the powder particles of the stream of powder are driven radially apart. Such an apparatus is shown, for example, in Federal Republic of Germany Pat. No. 20 30 388.